In device_ipx_changed() we remember the addresses for which it appears
that DAD failed. Later, on an idle handler, we process them during
queued_ip6_config_change().

Note that nm_plaform_ip6_address_sync() might very well decide to remove
some or all addresses and re-add them immidiately later. It might do so,
to get the address priority/ordering right. At that point, we already
emit platform signals that the device disappeared, and track them in
dad6_failed_addrs.

Hence, later during queued_ip6_config_change() we must check again
whether the address is really not there and not still doing DAD.
Otherwise, we wrongly claim that DAD failed and remove the address,
generate a new one, and the same issue might happen again.